# C++ attribute: fallthrough (since C++17)

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Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.

### Syntax

`[[fallthrough]]` |  |

### Explanation

May only be applied to a null statement to create a fallthrough statement ([[fallthrough]];).

A fallthrough statement may only be used in a switch statement, where the next statement to be executed is a statement with a case or default label for that switch statement. If the fallthrough statement is inside a loop, the next (labeled) statement must be part of the same iteration of that loop.

### Example

Run this code

    void f(int n)
    {
        void g(), h(), i();

        switch (n)
        {
            case 1:
            case 2:
                g();
                [[fallthrough]];
            case 3: // no warning on fallthrough
                h();
            case 4: // compiler may warn on fallthrough
                if (n < 3)
                {
                    i();
                    [[fallthrough]]; // OK
                }
                else
                {
                    return;
                }
            case 5:
                while (false)
                {
                    [[fallthrough]]; // ill-formed: next statement is not
                                     //             part of the same iteration
                }
            case 6:
                [[fallthrough]]; // ill-formed, no subsequent case or default label
        }
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 2406 | C++17 | [[fallthrough]] could appear in a loop nested inside the target switch statement | prohibited

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 9.12.6 Fallthrough attribute [dcl.attr.fallthrough]

- C++20 standard (ISO/IEC 14882:2020):

  - 9.12.5 Fallthrough attribute [dcl.attr.fallthrough]

- C++17 standard (ISO/IEC 14882:2017):

  - 10.6.5 Fallthrough attribute [dcl.attr.fallthrough]

### See also

C documentation for fallthrough

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